Microwave landing system

ABSTRACT

The invention provides independent guidance and monitoring to be integrated into an MLS landing system and also provides a scaled-down MLS landing guidance system useable at small airports. In both cases an added fixed beam precision guidance system is integrated into the MLS time sequence format. The fixed beam guidance system is based upon the concept of using paired fixed overlapping beams, sequentially radiated by different fixed antennas. One pair is oriented to left and right of the centerline of the runway, and the other pair is oriented above and below the desired glideslope. The first set of paired beams overlap at the centerline of the runway in such a manner that an aircraft approaching exactly on centerline will intercept equal signal intensities to indicate an on-course approach. Conversely when the aircraft is off-course to one side of the centerline, it will intercept a stronger signal intensity for the fixed guidance beam which is directed to that side of the centerline and a weaker signal intensity for the fixed beam which is directed to the other side of the centerline. Similar operation is provided by the overlapped guidance beams operating in the elevation mode.

BACKGROUND AND PRIOR ART

For many years the airports have been equipped with the InstrumentLanding System known as the ILS. However, this landing system in aboutto be replaced by a new Microwave Landing System, known as the MLS whichhas recently received virtually worldwide acceptance. The first orderfor the ground portion of the system MLS has recently been placed by theFAA.

In view of the essential nature of aircraft landings, often consideredto be the most critial of ordinary maneuvers, it is important to have amonitoring and backup system that provides an independent check duringin-flight approaches to insure the absolute reliability of the databeing provided by the principal landing system. This would be inaddition to the usual ground based monitors which are used for checkingcourse alignments, signal strengths, etc. of the signals radiated fromthe ground based landing system components. In the aircraft, other typesof navigation techniques are constantly in use to check on the accuracyof the landing system, but none provides the high degree of reliabilityrequired during final approach.

One possible technique for providing an independent landing monitorsystem (ILM) for the critical landing operation is to provide aduplicate ground installation and to compare in the aircraft theguidance data provided by both ground facilities and if such data agreeswithin prescribed limits to proceed with the landing.

In ILS systems, it has been impractical to provide a second identicalILS as a back-up monitor at the same airport, principally because of thelarge size of the ILS antennas. Moreover, even though the antennas aremuch smaller in MLS systems because of the higher frequencies used,about 5000 MHz, it would be impractical to provide a second identicalMLS at the same airport, because of the very high cost of the MLSsystems, about $500,000 each.

A second ILM technique is to use airborne equipment already installedfor other purposes to provide landing guidance data for such independentmonitoring purposes.

The weather radar, either modified or unmodified and in cooperation withground-based reflectors or beacons, has frequently been suggested forimplementing an ILM, especially since weather radar is used when flyingunder bad weather (IFR) conditions.

This weather radar ILM concept is taught in my U.S. Pat. No. 3,243,816,which uses the airborne weather radar together with ground installedpassive reflectors or alternatively with radar beacons of the frequencyshift type, to display guidance paths for landing, or for landingmonitoring purposes.

More recently, Assam in his U.S. Pat. No. 3,729,737 added a furtherteaching involving the detecting, by means of airborne radar, of pluraltilted reflectors to generate glideslope guidance patterns for ILMpurposes.

Gendreu et al in U.S. Pat. No. 4,103,300 suggests the use of weatherradar and a ground beacon system for ILM purposes. However, histechnique tends to require complex airborne instrumentation andtherefore cannot use a standard weather radar, which is a majordrawback.

My U.S. Pat. No. 4,429,312 overcomes the problems of requiring anon-standard weather radar by using a standard weather radar beacon forILM purposes in a time sequence mode of operation.

The problem with all of the above noted weather radar uses for ILMpurposes is that they require the use of a weather radar which may notbe installed on all aircraft of interest. What is required then is anILM that provides a completely independent monitoring function which isintegrated within the framework of the landing guidance system that isbeing monitored and which does not require the installation of addedairborne equipment. In particular this invention seeks to provide thistype of ILM capability in the present worldwide standard MLS landingsystem.

An ILM for monitoring the progress of each landing at an MLS site istherefore desirable, whereby an aircraft can obtain truly independentconfirmation of the MLS guidance data from ILM ground equipment which isintegrated with the MLS ground equipment. In such a system the airbornederived ILM data will be truly independent, but can be based upon theuse of the already installed MLS airborne equipment without requiringadded airborne equipment.

MLS is a sequentially functioning system which can provide up to 15different functions at different times in the MLS radiating sequence.These functions can be divided into two separate categories, onecategory providing guidance, and the other category providing to theaircraft data relating to that particular MLS installation, i.e.location of MLS equipment with respect to the runway, equipment status,type of services provided, etc. The MLS signal format currently includesboth an auxiliary guidance function, and an auxiliary data function (notyet fully specified). These auxiliary unspecified functions are toaccommodated future growth of the MLS system.

MLS is also a very flexible system wherein at some installations, as forexample at a very busy airport, essentially all functions may beprovided, whereas at smaller airports only some functions may beprovided.

Each MLS function, when it is provided, i.e. radiated, is accompanied byan associated preamble identification code. Since each function has suchan identification code, the functions can be radiated or provided atdifferent or randomly selected times, not necessarily in any particularsequence. Specific sequences are however recommended in the ICAO SARPS(Standards and Recommended Practices) for installations that provide aparticular combination of functions. In addition each particularfunction must be radiated at a certain minimum repetition rateconsistent with the service that function performs, i.e. azimuthapproach guidance must be provided at a rate consistent withaircraft/pilot response for a desired guidance performance.

The precision guidance functions of the MLS are provided by means of anarrow beam that scans the region in which precision guidance is beingprovided. The time between successive passages of the scanning guidancebeams past the airborne receiving antenna is precisely measured by theairborne precision timing circuit and used to provide the desiredangular guidance data.

The purpose of using scanned beams for localizer and elevationdeterminations in MLS, as distinguished from fixed beams as used in ILS,is to permit the approach and landing of aircraft along nonlinearcourses having greater flexibility than straight-line paths, i.e.permitting curved azimuth and elevation approaches which are deemedespecially useful at high traffic airports. Although a curved approachpath may be useful at some distance from touchdown, in order to be morecertain of a safe landing, the aircraft will usually fly the last, mostcritial, portion of the approach to touchdown along the usualnon-maneuvering straight-line centerline course. Moreover, during mostapproaches, the aircraft will still follow a relatively standardstraight-line glidepath, typically a 3° glideslope which is the same asused in ILS landings, prior to touchdown.

In addition to the use or radiation of the scanning beam to provideprecision guidance, the MLS guidance function may also include the useof sequentially radiated fixed beams.

These fixed beams serve two separate purposes. One purpose termed OCI(out of course indication) is to suppress false courses outside theestablished MLS guidance region. These false courses might be caused byside lobes of the precision scanning beams. False course suppression isaccomplished by radiating one or more fixed beams that provide greatersignal strength, by a prescribed amount, than the side lobes of thescanning beam in the area in which it is desired to suppress possiblefalse guidance courses. Up to six false course suppression beams can beradiated within the azimuth guidance function and up to two within theelevation guidance fuction.

The second purpose of using radiated fixed beams is to provide aclearance capability. Clearance beams are used in the MLS installationswhere the azimuth scanning beam does not scan the entire, normallyprescribed, precision azimuth guidance region of plus of minus 40° aboutthe runway centerline, but scans only a portion of that region. In suchinstallations these clearance beams are radiated left and right of thescanning beam precision coverage, but within the specified guidancecoverage region. Measurement of the amplitudes of such beams willprovide a fly/left, fly/right signal for use in the aircraft forintercepting the region in which precision proportional guidance isprovided by the scanning beam. Both the OCI and the clearance beams areradiated at prescribed times within the time allocated to the guidancefunction within which they might be utilized.

It can be noted therefore that MLS is a sequentially operating syustemthat can provide many different guidance functions in a very flexiblebuilding block configuration. In addition this flexibility is enhancedby providing auxiliary functions for unspecified future growthpotential. Precision guidance data is provided by accurate measurementsof the times when the scanning beam passes over the aircraft. Inaddition airborne amplitude measurements are also being made todetermine the intensities of sequentially radiated fixed beams that maybe utilized at some MLS installations to prevent false courses (OCIbeams) and to aid in the acquisition of the precision guidance beams(clearance beams).

THE INVENTION

This invention provides a landing system having integrated therein anindependent monitoring capability in which signals from two differentlyfunctioning landing systems are integrated into a single system toprovide both landing guidance and guidance monitoring. This is apreferred embodiment of the system in which the results of each of thedifferently functioning landing guidance systems are independentlyarrived at in the aircraft, compared in the aircraft, and the landingeither completed or aborted depending on whether or not the two resultsare in acceptable agreement. The two systems respectively comprise:First, a well known MLS time sequence which is a part of the standardMLS system and during which landing guidance beams are scanned and theairborne receiver determines the position of the aircraft based on thetimes when the scanning beams pass over the aircraft; Second, a separateand differently operating system in which paired fixed landing guidancebeams are radiated toward the aircraft at predetermined MLS times andthe aircraft receiver compares the relative intensities of the pairedsignals for position determining purposes. The latter amplitude-basedsystem, for azimuth guidance, uses paired fixed guidance beamssequentially radiated at different available MLS times, and directedrespectively to the left and right of the centerline, and overlapped atthe centerline in such a manner that an aircraft approaching along thecenterline will intercept equal signal intensities to indicate on-courseapproach. Conversely, when the aircraft is off-course to one side of thecenterline, it will intercept a stronger signal intensity at the time oftransmission of the fixed beam which is directed to that side of thecenterline, and a weaker signal intensity at the time of transmission ofthe fixed beam which is directed to the other side of the centerline. Asa result, an off-course condition is indicated by unbalance of theintensities of the two fixed guidance beam overlapping signals, the sideto which the aircraft is off-course being identified by strengthening ofthe signal radiated by that beam and weakening of the opposite fixedguidance beam signal. The times in which these fixed beams are radiatedwill be MLS available times which are not necessarily assigned for anyother purpose, and the signals radiated therein will be used forproviding independent confirmation of the final straight-line path totouchdown and will be independent of the main scanned precision guidancesignals of the MLS.

Although the above discussion has focused on the azimuth guidanceaspect, similar overlapping guidance beams can provide for operation inthe elevation mode to provide independent landing guidance for elevationmonitoring purposes.

The present invention can also provide fixed beam landing guidance whichwill be operative in the event of failure of the main MLS scanningsystem, or alternatively, which can provide a simplified MLS fixed beamlanding system that is suitable for use at small civil airfields thatcannot afford MLS scanning beam guidance for reasons of cost, or for useat certain tactical landing areas that cannot employ MLS scanning beamguidance because of the size and weight of required scanning beamantennas.

Both the MLS scanning beam landing guidance generation, and the fixedbeam landing guidance generation furnishing the monitoring capability,can be performed by receivers of the MLS scanning beam type since suchreceivers are already designed to perform both timing and amplitudemeasurements. In addition the data processing required for ILMmonitoring is well within the capability of modern microprocessorsalready designed for MLS system use, i.e. the required additional ILMdata processing beyond that required for conventional MLS purposes alonedoes not raise any serious problems in the airborne receiver design.

OBJECTS AND ADVANTAGES OF THE INVENTION

It is the principal object of the invention to provide an independentfixed guidance beam system for confirming the accuracy of the landingdata provided by the MLS landing system which is now approved forworldwide installation, and more particularly to provide a guidancesystem that will use the same format and repeating time sequence thatthe MLS uses and that will be compatible and functional with airborneMLS receiving equipment.

Another major object of the invention is to provide an independentmonitoring system that can be implemented either by using independentlyradiated overlapping fixed beams from the ground, or alternatively byusing either the MLS sector clearance signal beams or the OCI beamswhich would be slightly modified and reoriented to make them overlapappropriately at the centerline of the approach path.

It is a further major object of the invention to provide a system of thetype specified which lends itself to the convenient configuration of ascaled-down fixed beam guidance system which is compatible with airborneMLS equipment for use at minor airports not equipped with MLS, and whichwill provide guidance accuracy that approaches the guidance accuracy ofthe main MLS system but at a small fraction of the cost thereof. The useof such a financially feasible scaled-down system is attractive becausesmall airports do not require curved approaches to their runways sincetraffic is not heavy.

A further object of the invention is to permit the use of a simplifiedand reduced-size MLS ground station for military use, since the largeground station size required for MLS scanning beam systems is a majordrawback in many tactical operations. The military, for example, haveselected MLS for future use but to date, despite the expenditure oflarge sums of money, have not succeeded in obtaining an MLS groundstation that is adequately small and light-weight for tactical use.

Other objects and advantages of the invention will become apparentduring the following discussion of the drawings showing preferredembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a typical radiation sequence for the variousfunctions of a known MLS system;

FIG. 2 is a diagram showing typical radiation antenna patterns of aknown MLS system;

FIG. 3 is a diagram showing the sequence of transmissions within an MLSguidance function, and showing the corresponding signals as received atthe aircraft;

FIGS. 4A and 4B respectively shown a first embodiment including azimuthand elevation antenna radiation patterns for a fixed beam precisionguidance system;

FIGS. 5A and 5B show the relative amplitudes of signals as received atan aircraft which were radiated from the fixed beam precision guidancepatterns of FIG. 4;

FIGS. 6A and 6B show azimuth antenna radiation patterns respectivelyfrom a conventional MLS system, and from a modified MLS system whereinthe OCI patterns have been altered to provide a second embodiment of theinvention wherein both MLS scanned beam and fixed beam precisionguidance are provided at the same ground location;

FIGS. 7A and 7B show azimuth antenna radiation patterns respectivelyfrom a conventional MLS system and from a modified MLS system whereinthe left and right clearance patterns have been altered to provide analternative form of the second embodiment of the invention wherein bothMLS scanned beam and fixed beam precision guidance are provided at thesame ground location;

FIG. 8 is a diagram showing for a typical MLS system the angularpositions at which clearance beam signals are transmitted relative tothe beginning and end positions of the TO and FRO scanned MLS beams;

FIG. 9 is a diagram showing a third embodiment of this invention whereinfixed beam precision guidance signals are transmitted during thescanning intervals of the TO and FRO scanned MLS beams;

FIGS. 10A and 10B are block diagrams respectively showing, for azimuthand elevation, embodiments of ground and airborne MLS equipment modifiedto implement the present invention which combines both MLS scanning beamguidance and fixed beam precision guidance at the same integrated groundlocation;

FIGS. 11A and 11B are respectively ground-station azimuth and elevationradiation patterns for a system as shown in FIGS. 10A and 10B;

FIGS. 12 and 13 respectively are diagrams illustrating the outputs of anairborne receiver in response respectively to guidance signals receivedthereat from conventional MLS scanning beam guidance ground equipment,and from a system employing fixed beam precision guidance signalsradiated from the ground.

DESCRIPTION OF PREFERRED EMBODIMENTS OF INVENTION

The typical MLS system is a sequentially operating system which radiatesboth guidance functions and data functions at differentexclusively-occupied time intervals in a repeating sequence. Thefunctions that can be radiated are, respectively:

    ______________________________________                                        Basic Data Word 1                                                                            Approach Azimuth Guidance                                      Basic Data Word 2                                                                            High Rate Azimuth Guidance                                     Basic Data Word 3                                                                            Approach Elevation Guidance                                    Basic Data Word 4                                                                            Flare Elevation Guidance                                       Basic Data Word 5                                                                            Back Azimuth Guidance                                          Basic Data Word 6                                                                            Future-System Growth Guidance                                  Basic Data Word 7                                                             Basic Data Word 8                                                             Auxiliary Data                                                                ______________________________________                                    

The number of guidance functions that are radiated by any particular MLSinstallation depends on the guidance services which must be provided atthat ground location. There is also certain basic data that must beradiated from that installation. The above listed Auxiliary Data andFuture-System Growth Guidance functions and their associated timeintervals are reserved to accommodate future needs not yet specified.

Each of the above listed data or guidance functions has its ownidentification code within the preamble associated with that function,which code uniquely identifies the nature of the associated function tothe airborne equipment. Since the individual functions are thusidentified by their codes, it is not necessary that these functions beperformed in any particular sequence. The aircraft receiver andprocessor include capability for recognizing the function being radiatedto it using the preamble which precedes it, whereby the aircraftreceiver is enabled to utilise the information appropriately.

Referring now to the drawings, FIG. 1 shows a function radiatingsequence both for data and for azimuth and elevation guidance for asimplified MLS system. Appropriate basic data words are radiated asrepresented by data function blocks labeled 1 at times available betweenthe times of radiation represented by other function blocks such asblocks 2 for azimuth and blocks 3 for elevation guidance functions,respectively, these functions all being radiated at repetition ratesspecified by the MLS ground equipment. A suitable preamble, such as thepreamble 4, is shown in each function block at the start of the functionbeing radiated, and the preambles for the various functions are radiatedby the antennas at the guidance installations 5 and 6, the elevationinstallation 6 being located near the approach end of the runway and theazimuth installation 5 being located along the centerline at the far endof the runway. Specifically, the preamble for the azimuth guidancefunction 2 is radiated from an antenna with a pattern 8', FIG. 2, at theazimuth installation that provides coverage throughout the guidancecoverage region 8 of +-40° azimuth. The basic data words are generallyradiated together with their preambles using this same angular coverage.If the preamble associated with a particular function indicates that itis an azimuth guidance function, then the TO and FRO scanning guidancesignal associated with that preamble will be radiated by a narrow beamantenna with a pattern 9 which scans that same sector 8. The actualguidance data are obtained within the aircraft receiver by measuringtimes between successive passages of the scanning beam pattern 9 pastthe aircraft, as well known in the MLS art.

FIG. 3 in the vicinity of the bracket a shows in more detail theorganization of the radiation during an azimuth scanning function of theMLS ground system including a preamble 11, sector signals 15, and TO andFRO scanning time intervals 20 and 20'. The preamble 11 is radiatedfirst and includes a continuous wave 12 used for acquisition by theairborne receiver of the radio frequency carrier. This carrier isfollowed by a receiver reference time synchronization code 13. This isfollowed by a function identification code 14, in this case identifyingthe function as azimuth approach scanning guidance. These radiationsconstitute the preamble 11 of duration shown by the arrow 21.

The next radiated signals are the sector signals 15 which begin bytransmitting a ground station identification code 16 which lasts for theperiod identified by the arrow 22. Following the identification code,there is radiated a constant level test signal 17 with duration as shownby arrow 23. This signal is used in the airborne equipment by switchingairborne antennas during this interval 23 to determine which airborneantenna provides maximum signal and hence should be utilized thereafter.Subsequently, OCI signals 18 are radiated which are processed in thereceiver to determine whether the aircraft is on a false course, i.e.following a scanning beam side lobe as indicated by a received OCIsignal being stronger than the false scanning beam signal. Next, a TOscan test pulse 19 is radiated followed by a TO scanning beam, and thena FRO scanning beam followed by a FRO scan test pulse 19' is radiated,the test pulses being radiated immediately preceeding and following thetimes allocated to the TO scanning beam time 20 and the FRO scanningbeam time 20'.

As can be seen across the top of FIG. 3 in the vicinity of the bracketb, the receiver in the aircraft develops appropriate signals based onthe ground-radiations as just discussed. Specifically, as shown in FIG.3 for the azimuth scanning function, the preamble transmissions producein the receiver a preamble sequence 21, followed by the ground stationidentification code 22, followed by the airborne antenna selectionradiation 23. There are shown two OCI radiations 24 and 25 (out of sixOCI radiations that could be transmitted), followed by the groundradiated TO scan test pulse 26. The TO and FRO scanning guidance beams28 and 29, respectively, are then received in the aircraft at timesdepending on the location of the aircraft with respect to the centerlineof the runway, and finally the FRO scan test pulse 27 is radiated. Theclearance pulses are not shown in FIG. 3 because they would not be usedin the FIG. 2 configuration because the scanning beam scans the full+-40° region, not a sector of reduced width.

In order to implement the present invention so as to provide bothscanned beam guidance and fixed beam precision guidance integratedtherewith in a common system, the fixed beam precision guidance signalsmust be incorporated into the just-described MLS system in a compatiblemanner, so that a monitoring capability can be provided by comparing inthe airborne equipment the scanning beam guidance results and the fixedbeam guidance results.

It is an important object of the invention to integrate the fixed beamguidance system into the MLS in such a way that future MLS groundinstallations which are modified to implement this invention byincorporating fixed beam precision guidance capability will not provideincorrect data to earlier model MLS airborne systems which are not somodified. Even though achieving this objective is desirable, it is notabsolutely necessary because the added capability in later builtequipment could be restricted, at least for a time, to specialapplications which would not be utilized with the older MLS airborneequipment. For instance, the modified ground systems could initially berestricted to certain tactical ground installations for the battleenvironment, the use of which would be restricted to aircraft equippedwith updated tactical MLS type receivers that would be configured inaccordance with this disclosure.

For the purpose of illustrating the present inventive concepts whichintegrate fixed beam precision guidance into an MLS-type scanning beamsystem, three different embodiments will be discussed. The first ofthese embodiments is shown and described with reference to FIGS. 4 and 5and involves the radiation of overlapping fixed beams in both azimuthand elevation from additionally installed antennas which function withinvarious time intervals provided in the MLS sequence to accommodatefuture growth of the system. The second embodiment shows themodification of currently radiated MLS system beams to provide fixedbeam precision guidance, the modifications being of existing MLS OCIbeams as shown in FIG. 6, or being of existing left and right clearancebeams as shown in FIG. 7. The third embodiment shows the radiation fromadditionally installed antennas of my fixed beam precision guidancesignals during unused portions of intervals already assigned for otherMLS functions, for example as shown in FIG. 9 during the TO and FROscanning intervals of the MLS system.

Considering now the first embodiment shown and described with referenceto FIGS. 4 and 5, this embodiment uses the spare future guidancefunction, which is designed to accommodate future growth guidance needs,and radiates in its associated time interval my fixed beam guidancesignals. This embodiment uses the spare auxiliary data function toradiate data appropriate to accompany the fixed beam precision guidancesignals. This embodiment also employs paired separate antennasadditionally installed at the MLS ground location for radiating pairedfixed precision guidance beams along the approach path to providelanding guidance which is independent of the guidance provided by theMLS TO and FRO scanning beam identified by the reference numeral 9 inFIGS. 2 and 4A. These added fixed precision guidance beams are properlyidentified by their own preamble which defines their function to theairborne receiver and by auxiliary data transmitted therewith to enableproper processing of the signals received at the aircraft from the fixedguidance beams. In this embodiment the fixed beam precision guidancefunction is provided by added antennas 80 and 81, FIG. 10A, driven bythe ground equipment sequentially to radiate a left pattern 31 and aright pattern 32 with respect to the desired landing azimuth centerlinecourse 36. FIG. 4B shows the radiation of an elevational upper pattern33 and lower pattern 34 with respect to a desired glideslope usingantennas 89 and 90, FIG. 10B. Note that the azimuth beams 31 and 32partially overlap along the centerline 36 in FIG. 4A, and that the fixedguidance beams 33 and 34 for elevation partially overlap along thedesired glideslope 36' selected for monitoring in FIG. 4B. The beam 30shown in the latter figure comprises the UP and DOWN MLS scanning beamcovering the vertical arc 30'.

The fixed beam guidance function signals as received in the aircraft areshown in FIGS. 5A and 5B and include in each case a preamble 37containing information, including function identification, etc.,followed by paired fixed beam guidance signals respectively representingazimuth and elevation information for two different locations (35,35'and 36,36') of an approaching aircraft. The fixed beams 31, 32, 33 and34 in FIG. 4 are all transmitted with equal intensity along the approachpath. Since the paired beams respectively overlap equally along theselected glideslope 36' and centerline 36, an aircraft which isprecisely located therealong will receive equal intensities of pairedsignals from all four of these beams. Thus FIG. 5B shows the resultingsignals 38, 39, 40 and 41 in the aircraft receiver to be of equalamplitudes. However, if the aircraft strays from the desired path, thesignal from the beam which is directed more toward the side to which theaircraft has strayed will be strengthened while the signal from the beamdirected away from that side will be weakened. Thus FIG. 5A shows thatfor an aircraft position 35,35' left-of-center and above-glideslope, theleft signal 38' is stronger than the associated right signal 39', andthe upper signal 40' is stronger than the associated lower signal 44'.This unbalance in the signals 38',39' and 40',41' is processed in theaircraft receiver to deliver an appropriate output signal whoseamplitude indicates non-centered position of the aircraft to the left inazimuth and above the selected glideslope in elevation, because when theaircraft strays from the desired path, the signal strengthens on theside to which it has strayed, and weakens on the side from which it hasmoved away. Moreover, the degree to which straying off-course hasoccurred is proportionally indicated by the degree of unbalance of theamplitudes of the paired signals 38,39 and 40,41, to achieveproportional-guidance.

These output signals may be used either to provide auxiliary monitoringsignals which in the MLS system are used for comparison with similaroutputs based on MLS scanning beam guidance signals, or alternativelythe fixed beam guidance can replace the MLS scanning beam signalsentirely for small airport installations in order to provide at suchairports less complex and expensive landing capability which is stillcompatible with the receiver systems in aircraft having full MLScapability. Radiation of this fixed beam precision guidance andassociated data utilizes only a very small percentage of the auxiliarytime allocated in the MLS system to accommodate as yet undefined futuregrowth of the system. Since only a minor amount of this auxiliary timeallotment is needed for the fixed beam guidance function, time availablefor future growth is scarcely diminished.

At present it is normal practice to radiate the preamble with itsreference time code for a guidance function by using the same equipmentthat radiates the guidance beams themselves, as distinguished fromradiating the preamble and the associated guidance functions usingseparately located ground equipments. Thus for systems wherein theazimuth and elevation functions are co-located the transmission meansfor the preamble data functions can be especially economicallyinstalled. However nothing precludes the use of this embodiment in aninstallation such as is shown in FIG. 1 wherein the azimuth andelevation radiating equipments are located widely spaced along therunway.

Consideration will now be given to the second embodiment concept whichuses a somewhat different approach to the fixed-beam precision guidanceconcept. The different approach is illustrated by two different formsshown respectively in FIGS. 6 and 7. The second embodiment differs fromthe concept discussed in connection with FIGS. 4 and 5 in that, insteadof providing additional antenna means for performing the fixed beamprecision guidance functions as related above, the second embodimentalters the radiation patterns of existing MLS antennas to overlap theirbeams along the centerline and thereby achieve the desired fixed beamprecision guidance, without diminishing the normal functions of thosealready-existing antennas. FIG. 6A shows unmodified radiation patternsincluding left and right OCI beams, while FIG. 6B provides a first formof the embodiment which uses these OCI (Out of Course Indication)antenna beams of the MLS system by overlapping the beams at thecenterline 36. FIG. 7 provides a second form of the embodiment whichuses the right and left Clearance antenna beams of the MLS system,described hereinafter.

In the first form, FIG. 6A shows standard MLS OCI beams 43 and 44 usedto suppress the possibility of false course approaches by an aircraftwhich might be following a side lobe 45 of a scanning beam antennainstead of the main beam 9. When the aircraft is outside the +-40°scanning sector, it is considered to be outside of the guidance region8. These OCI beams 43 and 44 are radiated at such intensities that foran aircraft located outside the guidance beam region 8 their receivedamplitudes must be greater than the signals from any scanning beam lobeor clearance beam guidance lobe received, but for an aircraft locatedwithin the region 8 the OCI signal intensity must be at least 5 db lessthan the guidance signals. After an airborne receiver has sequentiallyreceived signals respectively identified by their times of occurencewithin the guidance function as being OCI signals, and in addition hasreceived clearance signals (from installations employing them) andscanning beam signals, then if the clearance or scanning beam guidancesignals are not greater by at least 5 db in intensity than the OCIsignals, the airborne signal processor warns the pilot that he is out ofthe guidance coverage region 8. Of course, the OCI signals are ofgreater intensity than the scanning beam signals 9 or clearance signals(if radiated) at all locations outside the azimuth coverage of theguidance region 8. Up to six azimuth and two elevation OCI beams areprovided for in various MLS systems. Nothing in the MLS OCIspecification criteria, therefore, precludes OCI beams from being usedto provide the present fixed beam precision guidance during the assignedOCI radiation times.

As shown in FIG. 6B, the OCI beams have been modified to provide theoverlapping contour shown as beams 46 and 47 which overlap the azimuthcenterline 36 at 48. When thus configured, the beams 46 and 47 can stillperform their normal OCI functions, while at the same time the portionsof the beams overlapping the centerline at 48 can give the same kind offixed beam precision guidance function as was discussed above withrespect to FIGS. 4 and 5. Such dual function OCI and fixed beam guidanceradiations would not be incorrectly interpreted by present dayunmodified MLS receivers since their fixed beam guidance capabilitieswould simply be disregarded thereby, while at the same time thereceivers would properly utilize the OCI information. On the other hand,future MLS receivers which are appropriately programmed by radiated datawords to take advantage of the dual OCI beam capabilities, would alsoobtain fixed beam precision guidance from these OCI beams.

It is recognised that in some airport locations having severe lateralmultipath problems it may be difficult to utilize the wide coveragefixed OCI beams 46 and 47 shown in FIG. 6B to provide precision fixedbeam guidance having adequate definition and freedom from lateralmultipath effects by overlapping the beams along the centerline 36.Nothing in the MLS specified criteria for generation of OCI beams,however, precludes the radiation during unused OCI time intervals ofnarrow precision fixed guidance beams 31, 32, 33 and 34 as shown inFIGS. 4A and 4B. Such beams would be ignored by presently existing MLSreceivers, but would be properly utilized by future MLS receivers havingthe proper programming enabled by auxiliary data words, for example, toprocess their precision fixed beam guidance data.

As an alternative form of this second embodiment of the invention whichin either form alters existing MLS radiated beams and uses them forfixed beam precision guidance, FIG. 7 serves to illustrate the conceptof modifying the standard MLS azimuth clearance beams 50 and 51 shown inFIG. 7A, by extending them to overlap along the centerline 36 of theapproach path to assume the beam shapes shown at 52 and 53 of FIG. 7B.As discussed with reference to FIG. 2, in the usual MLS system thescanning beam 9 provides coverage over the whole guidance region 8 whichextends +-40° of the centerline 36. However, in the scanning beamembodiment depicted in FIG. 7, the scanning beam 9' only providesprecision scanning beam proportional-guidance over a sector 49 extending+-10° each side of the centerline 36, such restricted angular coveragebeing appropriately noted in the radiated basic data words and being anexisting standard alternative form in the MLS system. The left and rightsector arcs 8" and 8'" which are located just outside the scanned region49, and which extend to the MLS specified limits of +-40°, are accordingto present MLS specifications covered by MLS clearance beams 50 and 51as shown in FIG. 7A. Measurements of the signal intensities of theclearance beam signals are used in the aircraft to provide appropriatefly/left or fly/right indications to the pilot to guide him to interceptthe main scanned guidance beam 9' in the sector 49. The criteria setforth in the MLS specifications for the clearance beams are: that thesignal intensity of clearance beam 50 for an aircraft located in sectorarc 8" must exceed the signal intensity of clearance beam 51 by 15 db,and must exceed the side lobes of the scanning beam signal 9' by 5 db;and in addition, that the signal intensity of clearance beam 50 must be5 db below the scanning beam signal intensity for an aircraft located at-10° along line 49a, i.e. at the negative edge of the scanning beamcoverage region 49. Similar specifications apply to clearance beam 51 insector 8'". It is therefore apparent that nothing in the MLSpecification regarding the clearance beams prevents these clearancebeams from being overlapped along the centerline 36 as shown at 52 and53 in FIG. 7B for the purpose of providing fixed beam precision guidancefor aircraft located within the +-10° guidance region 49.

FIG. 8 shows the standard MLS format for the clearance beams relative tothe +-10° scanning beam 9' which in the figure are shown four timesadjacent to the four outermost angular positions of the scanning beam9'. The four clearance beam pulses 55, 56, 57 and 58 are radiated attimes when the scanning beam 9' has reached its outermost TO and FROscan limits. For an aircraft position which is to the left of thecenterline 36, for instance as shown at 49a in FIG. 7A, the fly/rightpulse intensities 56 and 57 as received at the aircraft would be thesame as each other, but less (not illustrated) than the fly/left pulseintensities 55 and 58. For a centerline aircraft location 36, theintensities of all four pulses would be equal as shown in FIG. 8. Itshould be noted that in MLS systems where the scanning beam scans thefull 40° each side of center, so that clearance beams are not employed,FIG. 4A, they can still be added to the system for fixed beam precisionguidance purposes, and will be used for guidance monitoring purposes byupdated and properly programmed MLS receivers, while being ignored bypresently existing unmodified MLS receivers.

Clearance beams are not radiated with MLS elevation functions, andtherefore the use of clearance beams for providing fixed beam precisionguidance is only suitable for azimuth-guidance MLS monitoring orindependent azimuth guidance. Hence the use of clearance beams is notgenerally as useful a form of the present invention as is the use of OCIbeams.

Considering now the third embodiment of the present invention asdescribed with reference to FIG. 9, this embodiment radiates from newlyadded fixed beam precision guidance antennas during unused timeintervals within the guidance function signal format, as distinguishedfrom using auxiliary guidance function time intervals as provided in theMLS system to accommodate future growth of the system as exemplified bythe first embodiment, or from using presently existing radiated beamswithin already existing guidance functions which are also used for otherpurposes such as OCI or clearance beams.

This third embodiment employs, for instance, additional fixed beamprecision guidance antennas oriented according to FIG. 4, but whichradiate their fixed guidance beams at unused suitable times during theover-all interval of time allocated for the scanning beam function. Theuse of these times is somewhat along the lines suggested in the EneinU.S. Pat. No. 4,306,239. Such unused suitable times would be outside thetimes of actual beam scanning and/or clearance beam radiation, since theinterval of time allocated for the scanning beam is much longer than isrequired for normal scanning beam operation and/or clearance beamradiation. Normal scanning beam operation does not exceed +-40°, whereassufficient time is included in the interval to permit up to +-62° ofscanning. The time allocated for the scanning operation between the+-40° and the +-62° is thus normally available for other uses, such asthe radiation of fixed beam precision guidance pulses.

FIG. 9A shows the interval of time allocated to the scanning functionwithin the MLS time sequence, and FIGS. 9B and 9C show correspondingsignals received in the aircraft for azimuth guidance, assuming theradiation of azimuth fixed beam guidance pulses of the type discussed inconnection with FIG. 4A. As illustrated in FIG. 9B, the left fixedguidance beam is radiated at a time corresponding with -50° of the TOscan as shown at 60, and again at a time corresponding with -50° of theFRO scan as shown at 63. Likewise, the right fixed guidance beam isradiated at a time corresponding with +50° of the TO scan as shown at61, and again at a time corresponding with +50° of the FRO scan as shownat 62. The angular scale is shown in FIG. 9A. The relative intensity ofthe left fixed guidance beams 60 and 63 with respect to the intensitiesof the right fixed guidance beams 61 and 62 as shown in FIG. 9B, whencompared in the processor of the MLS receiver, serve to indicate thatthe aircraft is off the centerline 36 to the left. The differences ofthese intensities indicate how far off the centerline the aircraft islocated. Equal amplitudes of the intensities, i.e. of all four pulses60', 61', 62' and 63' as shown in FIG. 9C indicate that the aircraft isexactly on the centerline 36. Similar relative intensities wouldindicate position of the aircraft in elevation during a subsequentelevation scanning function. Auxiliary data words radiated inassociation with the fixed beam guidance pulses inform the aircraftreceiver of the portion of its program which should be used to processthe signals received in the aircraft from that that ground location.Furthermore, the signals radiated according to this third embodiment areradiated at times different from the times normlly used for radiatingclearance beam signals, or test pulses, or scanning beam signals, sothat presently existing MLS receivers will not erroneously interpretthese fixed beam precision guidance signals. Ample time is allocated inthe MLS scanning beam scanning time interval for this purpose.

Accordingly, it can be seen that there are many different practicalembodiments, including those herein described as well as others notdescribed, that can be used to radiate fixed beam precision guidancepulses for both azimuth and elevation guidance within the framework ofthe specified MLS format, which format is very flexible and has vastunused and/or unassigned time periods leaving ample time in which toradiate the fixed beam guidance signals as well as auxiliary data wordsto appropriately programmed MLS receivers, while avoiding erroneousresponses by presently existing unmodified receivers.

It may also be true that some presently existing unmodified MLSreceivers do not have the precision needed for utilizing the presentfixed beam guidance signals to generate precision landing guidance. Theproblem is that they may lack adequate capability for accuratelymeasuring the relative amplitude intensities of the fixed beam guidancesignals with such precision as would be required to match the accuracyof the scanning beam guidance, in view of the fact that existingreceiver amplitude measuring capability may be only sufficient toprovide false course suppression based upon signal strengths receivedfrom MLS OCI beams, or to provide fly/left, fly/right guidance basedupon measurements of signal strengths from clearance beams. In addition,signal processing software required for use in the receiver inconjunction with the fixed beam guidance function is not currentlyprogrammed into present models of MLS receivers. However, propermodification of current receivers to use the present fixed beam guidancefeatures is easily accomplished, including improvement of theircapability for signal intensity measuring and comparing, andimprovements to add the appropriate software. New receiver models canreadily incorporate the needed precision amplitude measuring capabilityand the appropriate software.

FIGS. 10A and 10B show integrated systems operative to provide bothscanning beam and fixed beam guidance according to this invention, FIG.10A showing an azimuth portion of the system and FIG. 10B showing anelevation portion of the system. The specific detailed embodimentdescribed herein is for a conventional split-site MLS system similar tothat of FIG. 1 but having fixed beam precision guidance integratedthereinto using OCI allocated time intervals, and providing eithermonitoring of the scanning beam guidance function or alternativelyproviding for independent fixed beam landing guidance for use at smallairports. In both FIGS. 10A and 10B an airborne receiver and dataprocessing means is illustrated which is in fact the same airborne unitalthough it is repeated in both diagrams. The azimuth ground equipmentshown in FIG. 10A includes a transmitter 70 which selectively feeds, viaa suitable switch unit 72, multiple antennas 74 through 81 whichrespectively have different radiation pattern shapes depending on thefunction that each is desired to perform. Although switch 72 is shownschematically as a rotary switch, it would comprise an electronicswitching unit in a practical installation. The position of the switch72, and its dwell time at each selectable position, is controlled by aprogrammed radiation control logic unit (RCLU) 71. The control unit 71also controls whether the transmitter 70, when connected to a particularantenna, transmits a continuous wave output or whether it transmits adata encoded function, for example, the reference time code orappropriate data words. Such encoding is provided by the data encoder 73when called for by the RCLU 71.

The elevation equipment shown in FIG. 10B similarly includes atransmitter 83 which selectively feeds, via a suitable switch unit 85,multiple antennas 87 through 90 which respectively have differentradiation pattern shapes depending on the function that each is desiredto perform. The position of the switch 85, and its dwell time on eachselectable position, is controlled by a programmed radiation controllogic unit (RCLU) 84 which also controls whether the transmitter 83,when connected to a particular antenna, transmits a continuous wavefunction or whether it transmits a data encoded function. Such encodingis provided by the data encoder 86 when called for by the RCLU 84.

The beams radiated by the various antennas provide signals received inthe air by airborne equipment (repeated in FIGS. 10A and 10B) whichincludes an antenna 92 coupled to an airborne receiver 93. The outputsignals from the receiver are delivered to an airborne data processor 94which performs all the programmed functions necessary to provideguidance, and which delivers guidance signals to drive landing guidancemeans which in this embodiment comprise a crossed needle landingindicator 95 which is of standard form.

The systems shown in FIGS. 10A and 10B provide the standard functionscomprising the currently accepted MLS system, but in addition can beutilized as set forth hereinafter to provide either scanning beamguidance compatibly combined with my novel fixed beam guidancemonitoring system, or alternatively to provide guidance using only myfixed beam guidance system, i.e. at airports which are small and havelow traffic levels and therefore do not require the more sophisticatedcurved approach paths that the scanning beams of the full MLS system canprovide.

As discussed previously, MLS is a very flexible system that can beinstalled in a wide variety of configurations, depending on the terrainat a particular site and the traffic volume, etc. The particulardetailed MLS embodiment discussed below with reference to FIGS. 11A and11B was chosen as representative of a typical installation and also asan illustration of previous discussions within this disclosure. Theazimuth portion of the MLS system shown in FIG. 10A is configured toprovide scanning beam coverage using the beam 9' shown in FIG. 11A toprovide a precision guidance region 49 of +-10°. Beams 50 and 51 provideclearance sector beams outside this +-10° region and extending guidanceto the limits of +-40°, defined by the sector 8. OCI beams 43 and 44provide false course suppression outside the guidance limits of +-40°.Beams 31 and 32 provide the fixed beam precision guidance of myinvention.

The elevation portion of the MLS system is illustrated by FIG. 10B andFIG. 11B, and is configured to provide scanning beam coverage using beam30 scanning the vertical arc 30', while the beams 33 and 34 provide thefixed beam precision guidance of the present invention.

A sequence of conventional MLS beam radiation from the ground antennasof FIG. 10 is normally as follows commencing with the azimuth equipment.As shown in FIG. 10A, a preamble is first transmitted from antenna 74with antenna pattern coverage corresponding to +-40° toward anapproaching aircraft in the forward guidance region 8 as shown in FIG.11A. The preamble 11, FIG. 3, includes an identification of the MLSfunction 14 being radiated which operates in the receiver to call up theproper processor program to process the data being radiated within thatsame guidance function from the ground installation. Sector beams 15 arethen radiated from the antenna 74, commencing with the groundinstallation identification code 16 (station identity) and the airborneantenna selection signal 17. Then the out-of-course OCI beams 18 aretransmitted from the azimuth antennas 75 and 76 to provide the beams 43and 44, FIG. 11A, to warn the pilot when he is outside the guidanceregion 8. The TO Test pulse 19 is then radiated via the antenna 74,followed by radiation of the left clearance beam 50 via antenna 78,which is then followed by the TO scanning beam 9' via the antenna 79,after which radiation of the right clearance beam 51 occurs via theantenna 77. After a pause, a second right clearance beam is radiated viathe antenna 77, followed by the FRO scanning beam 9' using the antenna79, which is then followed by radiation of a left clearance beam 50 bythe antenna 78. Radiation of the FRO test pulse 19' via the antenna 74terminates the MLS azimuth guidance function.

The azimuth function then gives way to the elevation function sequence.The elevation preamble is radiated from the antenna 87, FIG. 10B, withbroad coverage, and the elevation scanning beam 30 of FIG. 11B isradiated from the antenna 88. After the elevation beam have beenradiated, associated basic data words are radiated, followed by anotherelevation function. Then the system reverts to radiation of a nextazimuth fuction in the sequence. The airborne receiver and dataprocessor perform in accordance with conventional MLS practice.

The technique for including the radiation of my fixed beam precisionguidance functions in the FIG. 10 MLS system is as follows: Firstconsidering azimuth guidance, FIG. 10A, there are six OCI time intervalsavailable for radiation of OCI beams in the present MLS specification.It is conventional to radiate only two OCI beams 43 and 44, FIG. 11A, injust two of these six intervals, leaving four unused OCI intervalsavailable. The present fixed beam precision guidance beams can thereforebe radiated during unused OCI intervals by programming the logic controlunit 71 to transmit sequentially via the switch 72 and antennas 80 and81 my fixed beam precision beams having beam patterns 31 and 32 whichoverlap along the centerline 36 as shown in FIG. 11A. In addition thelogic control unit 71 initiates the transmission of auxiliary data wordsusing the auxiliary data function and the antenna 74 and switch 75, atappropriate times in the overall MLS radiating sequence, which datawords identify the nature of the fixed beam precision function.

Referring now to the elevation equipment, FIG. 10B, in a similar mannerthe logic control unit 84 is programmed to energize the transmitter 83during two elevation OCI time intervals and to connect it via the switch85 to the antennas 89 and 90 to radiate fixed beam precision guidancebeams with patterns 33 and 34 which overlap the desired glideslope 36'selected for monitoring, FIG. 11B. Presently existing MLS receivers, notconfigured in accordance with this invention, would ignore these fixedbeam precision guidance beams 33 and 34, which are radiated during OCItime intervals, as previously discussed. Conversely, an MLS receiverprogrammed in accordance with this invention would receive auxiliarydata words and appropriately use them to process fixed beam precisionguidance beams to generate appropriate guidance indications.

FIG. 12 shows the output response by an MLS receiver and processor toazimuth MLS scanning beam guidance beam received in response to scanningbeam 9' as shown in FIG. 11A. The illustrated airborne response to theMLS scanned proportional guidance system includes a curve P of theprocessor output voltage for various different angular locations of theaircraft on both sides of the 0° centerline 36, plotted horizontally.For an aircraft position at 0°, on the centerline, the output will bezero, meaning that the azimuth needle A of the indicator 95, FIG. 10,will be centered. As displacement of the aircraft off the centerlineincreases, the output on curve P will linearly increase needle movementfor displacements up to 10° on each side of center, although theover-all curve is shown for deviations up to 40° each side of thecenterline. The response of the FIG. 12 curve beyond the linear portionP, which is the limit of scanning beam proportional coverage, iscontrolled by the clearance beams 50 and 51 to provide constantamplitude fly/left and fly/right signals PL and PR.

FIG. 13 shows a curve K which is similar to curve P in FIG. 12 inover-all appearance, but represents my fixed beam guidance systemresponse. Less of the contour of the curve K beyond the centerline 36 islinear, the linear portion encompasing less than about 5° each side ofthe centerline, for instance for aircraft approaching between the points96 and 97 as shown in FIG. 11A. However, useful guidance is still givenin the zones between the 5° and the 10° displacements off centerline 36.The clearance beams are also provided by the antennas 77 and 78 of FIG.10A to produce Fly-Left or Fly-Right indications as shown at KR and KL,just as in the case of FIG. 12. A fixed beam guidance curve forelevation (not shown) would have a contour similar to the azimuth curvegiven in FIG. 13. A comparison of FIGS. 12 and 13 shows that ascaled-down system which uses only my fixed beam guidance system, andeliminates scanning beams, gives results which are comparable with thoseof the full MLS system, and which are quite adequate for small airportuse. It is pertinent to note that the ILS landing system, as used formany years at large airports, provides linear guidance in itsproportional display for only about 21/2° each side of the centerline,with the needles of the cockpit display pegged for positions beyond21/2° from the centerline to provide Fly-Left and Fly-Right instructionsto the pilot.

The above discussions show that the standard MLS system, the MLS systemaugmented to include my fixed beam monitoring system, and thescaled-down system using my fixed beam guidance in place of scanningguidance signals are all mutually compatible, and can all beinterchangeably used at airports around the world, the different systemsmerely requiring appropriate programming of the ground radiation controllogic 71 and the ground data encoder 73 to indicate which type of systemis at a particular location, and requiring appropriate programming inthe airborne data processor 94 to cooperate with the different groundinstallations. The discussions also show that my fixed beam system,while using the same ground transmitter 70 as the MLS system, usesdifferent antennas and operates differently and at different times inthe MLS radiation sequence, and therefore operates independently to alarge extent of the MLS scanning beam system when serving to monitor thelatter.

This invention is not to be limited to the exact embodiments shown inthe drawings, for obviously, changes may be made within the scope of thefollowing claims.

I claim:
 1. The method of providing MLS aircraft landing guidance andindependently monitoring the accuracy of that MLS guidance for aprescribed approach path by transmitting from ground based antenna meansin a repeating sequence of allocated MLS time intervals plural diverseguidance functions respectively including scanning beam precisionlanding guidance functions interspersed with fixed-beam precisionguidance functions, including the steps of:(a) scanning said scanningbeam in first and second opposed directions across an approach regionincluding the prescribed approach path to provide said scanning beamfunction, the scanning beam being transmitted within its own allocatedMLS time intervals in the sequence and starting and stopping on oppositesides of the prescribed approach path; (b) transmitting duringnon-interfering MLS time intervals fixed-beam guidance beams pairedalong opposite sides of the prescribed approach path such that theradiation patterns of the paired beams overlap the prescribed approachpath by the same amount to provide said fixed beam function, wherebytheir intensities as measured along the prescribed approach path aremutually equal; (c) receiving in an approaching aircraft saidtransmitted guidance beams to provide received signals; (d) processingthe signals received from the scanning beam guidance function to providea first output signal which varies in proportion to the time between thereception in the aircraft of the scanning beam passing in said firstdirection and the reception of the scanning beam passing in the seconddirection at the present location of the aircraft, and using such firstoutput signals to generate scanning beam guidance signals with respectto the prescribed approach path; (e) processing the signals receivedfrom the paired fixed-beam guidance function by comparing their relativeamplitudes to provide a second output signal which varies in proportionto the relative strengths thereof as received at the present location ofthe aircraft, and using such second output signals to generate fixedbeam guidance signals with respect to the prescribed approach path; and(f) comparing said first output signals and said second output signals,and providing for aircraft guidance said scanning beam guidance signalswhen the first and second output signals substantially agree.
 2. Themethod as claimed in claim 1, including the steps of transmitting inassociation with said guidance functions identifying data functionsoperative to indicate the type of guidance functions being transmitted,whereby to indicate in the aircraft how received signals should beprocessed.
 3. The method as claimed in claim 2, including the steps ofalternatively transmitting said scanning beam guidance functions andsaid fixed-beam guidance functions in azimuth orientation and inelevational orientation.
 4. The method as claimed in claim 2, whereinthe system includes MLS OCI (out-of-course indication) beams transmittedin out-of-course sectors other than said approach region at MLSspecified amplitudes for the OCI beams whose amplitudes are greater thanany guidance beam amplitude in the out-of-coverage sectors and which areat least 5 dB less than the scanning beam amplitude within the scanningbeam region, and said OCI beams being shaped and directed to enter theapproach region and overlap with equal beam intensity said prescribedapproach path, said paired OCI beams providing said fixed-beam guidancesignals in the aircraft.
 5. The method as claimed in claim 2, whereinfor azimuth guidance the system includes MLS clearance beams transmittedin clearance sectors adjacent to the scanning beam approach region atMLS specified amplitudes such that each clearance beam amplitude exceedsthe other clearance beam amplitude in the clearance sector of the otherclearance beam by 15 dB and it exceeds the side lobe amplitudes of thescanning beam by at least 5 dB in its clearance sector and such thateach clearance beam amplitude is at least 5 dB below the scanning beamamplitude at the edge of the scanning beam region, and said clearancebeams entering the approach region and overlapping said prescribedapproach path with equal beam intensity and providing said fixed-beamguidance signals in the aircraft.
 6. The method as claimed in claim 2,wherein said fixed beam guidance beams are transmitted respectivelywithin MLS intervals of time allocated to the transmission of the MLSscanning beam function but outside times actually used for the MLSscanning beam transmissions.
 7. The method as claimed in claim 2,wherein the MLS includes an auxiliary guidance function time intervalallocated for future MLS system growth, and wherein each fixed-beamguidance beam is transmitted during an MLS auxiliary guidance functiontime interval.
 8. In an MLS landing system having ground locatedtransmitting means and having MLS airborne receiving and processingmeans and having a repeating sequence of time intervals foraccommodating plural diverse guidance functions which are transmitted inassociation with data identifying the type of function beingtransmitted, the possible guidance functions including scanning beamguidance functions comprising scanning beams scanned in first and seconddirections across a region which an aircraft is approaching for landingand including fixed beam guidance functions paired to overlap within theapproach region along a prescribed approach path where their mutualintensities are equal, the method of providing instrument landingguidance within aircraft including the steps of:(a) receiving guidancefunction beams in the aircraft and providing output signals basedthereon; (b) processing the data associated with the output signals todetermine the type of guidance function, as between scanning beamguidance function only, fixed beam guidance function only, and bothscanning beam and fixed beam guidance functions; and (c) processing theoutput signals appropriately as determined by the data associatedtherewith, and providing in the aircraft guidance signals which arebased upon a single type of guidance function when only one type ofguidance function is being transmitted, and which are based upon acomparison of both types of guidance functions when both types ofguidance functions are being transmitted.
 9. The method as claimed inclaim 8, including the steps of alternately transmitting guidancefunctions in azimuth orientation and in elevational orientation, eachaccompanied by associated identifying data.
 10. The method as claimed inclaim 8, including the steps of transmitting during said repeating timesequences sector and preamble data for providing ground stationidentification and for use in the aircraft for signal acquisition andtiming purposes.
 11. The method as claimed in claim 8, wherein thesystem includes MLS OCI (out-of-course indication) beams transmitted inout-of-course sectors other than said approach region at MLS specifiedamplitudes for the OCI beams whose amplitudes are greater than anyguidance beam amplitude in the out-of-course sectors and which are atleast 5 dB less than the scanning beam amplitude when radiated withinthe scanning beam region, and said OCI beams being shaped and directedto enter the approach region and overlap with equal beam intensity saidprescribed approach path, said paired OCI beams providing saidfixed-beam guidance signals in the aircraft.
 12. The method as claimedin claim 8, wherein for azimuth guidance the system includes MLSclearance beams transmitted in clearance sectors adjacent to thescanning beam approach region at MLS specified amplitudes such that eachclearance beam amplitude exceeds the other clearance beam amplitude inthe clearance sector of the other clearance beam by 15 dB and it exceedsthe side lobe amplitudes of the scanning beam when radiated by at least5 dB in its clearance sector and such that each clearance beam amplitudeis at least 5 dB below the scanning beam amplitude when radiated asmeasured at the edge of the scanning beam region, and said clearancebeams entering the approach region and overlapping said prescribedapproach path with equal beam intensity and providing said fixed-beamguidance signals in the aircraft.
 13. The method as claimed in claim 8,wherein said fixed beam guidance beams are transmitted respectivelywithin MLS intervals of time allocated to the transmission of the MLSscanning beam function but outside times actually used for the MLSscanning beam transmissions.
 14. The method as claimed in claim 8,wherein the MLS includes an auxiliary guidance function time intervalallocated for future MLS system growth, and wherein each fixed-beamguidance beam is transmitted during an MLS auxiliary guidance functiontime interval.
 15. Apparatus for providing MLS aircraft landing guidanceand independently monitoring the accuracy of that guidance for anaircraft approaching along a prescribed path in an approach region, theapparatus transmitting from ground station means to airborne receivermeans during repeating sequences of allocated MLS time intervals pluraldiverse guidance functions respectively including scanning beam guidancefunctions interspersed with fixed-beam guidance functions, the apparatuscomprising:(a) at said ground station means, transmitter meansselectively connected by radiation control logic means to ground basedantenna means and operative for transmitting a scanning beam guidancefunction in its own time intervals which is radiated into said approachregion and including a scanning beam scanning in first and seconddirections across said approach region, and for transmitting duringnon-interfering time intervals fixed-beam guidance functions includingpaired beams directed along opposite sides of the aircraft approach pathsuch that the paired beams overlap said prescribed approach path by thesame amount, whereby their beam intensities as measured along the pathare mutually equal; and (b) in said aircraft, receiver means coupled toprogrammed data processor means connected to landing guidance means, thereceiver means being operative to receive signals based on guidance beamfunctions, and the processor means being operative to process thereceived signals based on the scanning beam function to provide a firstoutput signal which varies in proportion to the time between thereception of the scanning beam passing in the first and seconddirections at the present location of the aircraft, and the processormeans being operative to process received signals based on thefixed-beam guidance function by comparing their relative amplitudes toprovide a second output signal which varies in proportion to therelative strengths of paired fixed-beam signals at the present locationof the aircraft, and the processor means being operative to compare thefirst and second output signals and to deliver to the landing guidancemeans a guidance signal based on the first output signal when said firstand second output signals substantially agree.
 16. The apparatus asclaimed in claim 15, wherein said transmitter means includes dataencoder means enabled by the radiation control logic means to deliver tothe transmitter means identifying data for radiation to the aircraft toindicate the types of guidance functions being transmitted, and saidairborne data processor means being responsive to the nature ofidentifying data received and being rendered operative thereby toappropriately process received signals.
 17. The apparatus as claimed inclaim 16, wherein said radiation control logic means is alternativelyactuated by said transmitter means to radiate said scanning beamguidance function and said fixed-beam guidance function in azimuthorientation and in elevational orientation, each associated withtransmitted identifying data.
 18. The apparatus as claimed in claim 16,wherein said radiation control logic means is operative to actuate thetransmitter means and antenna means to radiate MLS OCI (out-of-courseindication) beams transmitted in out-of-course sectors other than saidapproach region at MLS specified amplitudes of the OCI beams whichamplitudes are greater than any guidance beam amplitude in theout-of-course sectors and which are at least 5 dB less than the scanningbeam amplitude within the scanning beam region, and means for shapingand directing said OCI beams to enter the approach region and overlapwith equal beam intensity said prescribed approach path, said paired OCIbeams providing said fixed-beam guidance signals in the aircraft. 19.The apparatus as claimed in claim 16, wherein for azimuth guidance saidradiation control logic means is operative to actuate the transmittermeans and antenna means to radiate MLS clearance beams adjacent to thescanning beam at MLS specified amplitudes such that each clearance beamamplitude exceeds the other clearance beam amplitude in the clearancesector of the other clearance beam by 15 dB and it exceeds the side lobeamplitudes of the scanning beam by at least 5 dB in its clearance sectorand such that each clearance beam amplitude is at least 5 dB below thescanning beam amplitude at the edge of the scanning beam approachregion; and means for shaping and directing said clearance beams toenter the approach region and overlap with equal beam intensity at saidprescribed approach path and provide said fixed-beam guidance signals inthe aircraft.
 20. The apparatus claimed in claim 16, wherein saidradiation control logic means is operative to actuate the transmittermeans and antenna means to radiate the overlapped fixed-beam guidancebeams within intervals of time allocated to the transmission of the MLSscanning beam function but outside times actually used for the MLSscanning beam transmissions.
 21. The apparatus claimed in claim 16,wherein the MLS includes an auxiliary guidance function time intervalallocated to future growth of the MLS system, and wherein said radiationcontrol logic means is operative to actuate the transmitter means andantenna means to radiate the fixed beam guidance beams during MLSauxiliary guidance function time intervals.
 22. In an MLS landing systemhaving multiple differently functioning ground stations for providinglanding guidance for an aircraft approaching a station along aprescribed path in an approach region and wherein the ground stationtransmits diverse aircraft guidance functions to airborne receiver meansin a repeating sequence of allocated MLS time intervals and transmitswith said guidance functions associated data functions defining thetypes of guidance functions transmitted at that MLS ground station,improved MLS apparatus wherein:(a) the various MLS ground stationscomprise transmitter means selectively connected by radiation controllogic means to ground based antenna means which are selectively operablefor transmitting at least one of diverse precision guidance functions inallocated time intervals in said sequence, which functions include ascanning beam guidance function radiated into said approach region andincluding a scanning beam scanned in first and second directions acrosssaid approach region, and which functions include fixed-beam guidancefunctions having paired beams directed along opposite sides of theaircraft approach path such that the paired beams overlap saidprescribed approach path by the same amount, whereby their beamamplitudes as measured along the path are mutually equal; and (b) theairborn MLS receiver means include means operative to receivetransmitted functions and include programmed data processor meansconnected to the receiver means and operative in response to receiveddata signals to process received guidance signals in the mannerdetermined by associated data signals; the processor means beingoperative to process precision scanning beam signals to provide outputsignals which vary in proportion to the time between the passing of thescanning beams in said first and second directions at the presentlocation of the aircraft; and the processor means being operative toprocess paired fixed-beam guidance signals by comparing their relativeamplitudes to provide output signals which vary in proportion to therelative strengths of the fixed-beam received signals at the presentlocation of the aircraft, and landing guidance means connected to theprocessor means and operative to receive a guidance signal based on oneof said output signals; (c) said processor means delivering to thelanding guidance means guidance signals which are based upon a singletype of guidance function when only one type of guidance function isbeing transmitted, and which are based upon a selected one of theguidance signals after a comparison of both types of signals when bothtypes of guidance functions are being transmitted.
 23. The apparatus asclaimed in claim 22, wherein said radiation logic control means isoperative to actuate the transmitter means and the antenna meansalternatively to radiate said guidance functions in azimuth orientationand in elevational orientation, each in association with identifyingdata.
 24. The apparatus as claimed in claim 22, wherein said radiationcontrol logic means is operative to actuate the transmitter means andantenna means to radiate MLS OCI (out-of-course indication) beamstransmitted in out-of-course sectors other than said approach region atMLS specified amplitudes of the OCI beams which amplitudes are greaterthan any guidance beam amplitude in the out-of-course sectors and whichare at least 5 dB less than the scanning beam amplitude within thescanning beam region, and means for shaping and directing said OCI beamsto enter the approach region and overlap with equal beam intensity saidprescribed approach path, said paired OCI beams providing saidfixed-beam guidance signals in the aircraft.
 25. The apparatus asclaimed in claim 22, wherein for azimuth guidance said radiation controllogic means is operative to actuate the transmitter means and antennameans to radiate MLS clearance beams adjacent to the scanning beam whenradiated at MLS specified amplitudes such that each clearance beamamplitude exceeds the other clearance beam amplitude in the clearancesector of the other clearance beam by 15 dB and it exceeds the side lobeamplitudes of the scanning beam when radiated by at least 5 dB in itsclearance sector and such that each clearance beam amplitude is at least5 dB below the scanning beam amplitude when radiated at the edge of thescanning beam approach region; and means for shaping and directing saidclearance beams to enter the approach region and overlap with equal beamintensity at said prescribed approach path and provide said fixed-beamguidance signals in the aircraft.
 26. The apparatus claimed in claim 22,wherein said radiation control logic means is operative to actuate thetransmitter means and antenna means to radiate the overlapped fixed-beamguidance beams within intervals of time allocated to the transmission ofthe MLS scanning beam function but outside times actually used for theMLS scanning beam transmissions.
 27. The apparatus claimed in claim 22,wherein the MLS includes an auxiliary guidance function time intervalallocated to future growth of the MLS system, and wherein said radiationcontrol logic means is operative to actuate the transmitter means andantenna means to radiate the fixed beam guidance beams during MLSauxiliary guidance function time intervals.